Atomistic mechanisms of crack nucleation and propagation in amorphous silica

Abstract

This paper presents an atomistic understanding of the nanoscale processes that govern crack nucleation and propagation in amorphous silica using a combination of theoretical calculations and molecular dynamics simulations with the ReaxFF potential. We show that both crack nucleation and propagation are governed by chainlike nanoscale virial stress-fibers formed by an intricate mixture of Si and O atoms. The stress-fibers are of a few nanometers in length, aligned parallel to the loading direction, and spatially localized at the evolving crack front at the nucleating site or a propagating crack tip. They form and break continuously during the crack nucleation and propagation process and are responsible for localizing stress at the crack nucleation site or propagating crack front. As soon as the stress-fibers reach a critical density the material starts nucleating cracks or leads to the propagation of initial crack. Additionally, the virial stress fields in the domain is highly heterogeneous and species-dependent---and the O and Si atoms play fundamentally distinct roles throughout the deformation process. With increased loading, heterogeneity in virial stress for the Si atoms goes up, whereas for the O atoms it goes down. Furthermore, from the virial and Hardy estimates of atomic stress, it is found that the stress field emanating from the crack tip decays as $1/r$. Also, presence of holes or pores within the interaction distance of a crack tip intensifies the stress state of the stress-fibers near the crack front leading to an improved effective toughness compared to the situation where the pore is far from the crack tip. Nucleation and propagation of cracks are strictly mediated by a complex admixture of localized bond rupture processes across a set of interacting stress-fibers. The details of the atomistic process regulating the underlying mechanisms are undetectable from the macroscopic stress-strain data.